(1) Field of the Invention
The present invention relates to an optical pickup apparatus which has a holographic optical element and a photodetector which are provided in common for light beams with different wavelengths used to access different types of optical storage media. (2) Description of the Related Art
Currently, various types of rewritable optical disk drive are known, for example, a write-once optical disk drive which accesses a CD (compact disk recordable) and a rewritable optical disk drive which accesses a DVD (digital video disk). An optical pickup device of the write-once optical disk drive reads data from the CD, and can write data to the CD once only. An optical pickup device of the rewritable optical disk drive reads data from the DVD, and can write or erase data to the DVD many times.
Generally, a standard DVD has the recording surface under a transparent substrate which is about 0.6 mm thick, and a standard CD has the recording surface under a transparent substrate which is about 1.2 mm thick. In a DVD pickup device, a laser diode which emits a laser beam having a wavelength of 650 nm is used as the light source to access the DVD. In a CD pickup device, a laser diode which emits a laser beam having a wavelength of 785 nm is used as the light source to access the CD.
As disclosed in Japanese Utility Model Publication No.7-3461, an optical pickup apparatus for recording or reproducing of information of one of a first optical disk and a second optical disk in a shared manner is known. As described above, the first and second optical disks have the transparent substrates which are different in thickness.
In the optical pickup apparatus of the above publication, first and second laser sources selectively emit one of first and second laser beams, the first and second laser beams being different in wavelength, the wavelengths of the first and second laser beams being appropriate for accessing the first and second optical disks respectively. A reflection-beam separator which is configured with a prism of a certain type receives a reflection beam of a light spot from one of the first and second optical disks which is actually illuminated, and directs the reflection beam in one of predetermined directions depending on the wavelength of the reflection beam.
Further, in the optical pickup apparatus of the above publication, a first photodetector is provided to receive the reflection beam (having the wavelength of the first laser beam) from the reflection-beam separator, and to output a signal indicative of an intensity of the received reflection beam. A second photodetector which is provided separately from the first photodetector receives the reflection beam (having the wavelength of the second laser beam) from the reflection-beam separator, and outputs a signal indicative of an intensity of the received reflection beam.
In the optical pickup apparatus of the above publication, a focusing error signal and a tracking error signal can be generated based on the signal output by a corresponding one of the first and second photodetectors. Hence, the recording or reproducing of information of one of the first optical disk and the second optical disk can be achieved by the optical pickup apparatus of the above publication.
However, the optical pickup apparatus of the above publication must be configured with the first and second photodetectors which are provided independently of each other. The configuration of this apparatus is comparatively complicated, and it is necessary to provide a separate signal detection circuit for each of the first and second photodetectors. This makes the conventional optical pickup apparatus bulky and expensive, and it is difficult to achieve the manufacture of a small-size optical pickup apparatus with low cost.
An object of the present invention is to provide an improved optical pickup apparatus in which the above-described problems are eliminated.
Another object of the present invention is to provide an optical pickup apparatus which is configured in a simple structure including the reflection-beam separator and the photodetector, in order to enable the manufacture of an inexpensive, small-size optical pickup apparatus.
Still another object of the present invention is to provide an optical pickup apparatus which is configured with an inexpensive, thin-film reflection-beam separator to direct the reflection beam in one of predetermined directions depending on the wavelength of the reflection beam, in order to enable the manufacture of an inexpensive, small-size optical pickup apparatus.
The above-mentioned objects of the present invention are achieved by an optical pickup apparatus for recording or reproducing of information of one of a first optical disk and a second optical disk in a shared manner, the first and second optical disks having transparent substrates different in thickness, the optical pickup apparatus including: first and second light sources which selectively emit one of first and second light beams, the first and second light beams being different in wavelength, the wavelengths of the first and second light beams being appropriate for accessing the first and second optical disks respectively; a coupling lens which converts a corresponding one of the first and second light beams from the first and second light sources into a collimated beam; an objective lens forming a light spot on a corresponding one of the first and second optical disks by focusing the collimated beam; a holographic optical element which receives a reflection beam of the light spot from one of the first and second optical disks and provides holographic effects on the reflection beam so as to diffract the reflection beam in predetermined diffracting directions depending on the wavelength of the reflection beam; and a photodetector which receives the reflection beam from the holographic optical element at light receiving areas of the photodetector and outputs signals indicative of respective intensities of the received reflection beam at the light receiving areas, so that a focusing error signal and a tracking error signal are generated based on the signals output by the photodetector.
The above-mentioned objects of the present invention are achieved by an optical pickup apparatus for recording or reproducing of information of one of a first optical disk and a second optical disk in a shared manner, the first and second optical disks having transparent substrates different in thickness, the optical pickup apparatus including: first and second light sources which selectively emit one of first and second light beams, the first and second light beams being different in wavelength, the wavelengths of the first and second light beams being appropriate for accessing the first and second optical disks respectively; a coupling lens which converts a corresponding one of the first and second light beams from the first and second light sources into a collimated beam; an objective lens forming a light spot on a corresponding one of the first and second optical disks by focusing the collimated beam; a holographic optical element which receives a reflection beam of the light spot from one of the first and second optical disks and provides holographic effects on the reflection beam so as to diffract the reflection beam in predetermined diffracting directions depending on the wavelength of the reflection beam; and a photodetector which receives the reflection beam from the holographic optical element at light receiving areas of the photodetector and outputs signals indicative of respective intensities of the received reflection beam at the light receiving areas, so that a focusing error signal and a tracking error signal are generated based on the signals output by the photodetector, wherein the optical pickup apparatus has a common optical path for the first and second light beams, and the coupling lens and the objective lens are arranged such that both an optical axis of the coupling lens and an optical axis of the objective lens are on the common optical path, and wherein the holographic optical element is arranged on the common optical path and configured with a polarization hologram and a quarter-wave plate, the polarization hologram having diffracting effects depending on polarizing directions of the reflection beam, and the quarter-wave plate being arranged on the common optical path such that the quarter-wave plate is placed on an optical-disk side of the polarization hologram, and wherein the polarization hologram includes: a transparent substrate; a birefringence layer of a polymer material which is provided on the transparent substrate in a periodic grating pattern, the birefringence layer having different refractive indexes for two orthogonal polarizing directions of the reflection beam; and an isotropic overcoat layer which is provided to enclose the birefringence layer therein, the polarization hologram diffracting the reflection beam in the predetermined diffracting directions depending on the wavelength of the reflection beam.
A preferred embodiment of the optical pickup apparatus of the present invention is configured in a simple structure including the holographic optical element and the single-piece photodetector. The holographic optical element receives the reflection beam of the light spot from the corresponding one of the first and second optical disks and provides the holographic effects on the reflection beam so as to diffract the reflection beam in the predetermined diffracting directions depending on the wavelength of the reflection beam. The photodetector receives the reflection beam from the holographic optical element at light receiving areas of the photodetector and outputs signals indicative of respective intensities of the received reflection beam at the light receiving areas, so that a focusing error signal and a tracking error signal are generated based on the output signals of the photodetector. Therefore, the optical pickup apparatus of the present invention is effective in enabling the manufacture of an inexpensive, small-size optical pickup apparatus.
Another preferred embodiment of the optical pickup apparatus of the present invention is configured with an improved holographic optical element to direct the reflection beam in one of predetermined directions depending on the wavelength of the reflection beam. The improved holographic optical element is arranged on the common optical path and configured with the polarization hologram and the quarter-wave plate, and the polarization hologram includes the birefringence layer of the polymer material provided on the transparent substrate in the periodic grating pattern, the birefringence layer having different refractive indexes for the two orthogonal polarizing directions of the reflection beam. Hence, the improved holographic optical element is an inexpensive, small-size reflection-beam separator which diffracts the reflection beam in the predetermined diffracting directions depending on the wavelength of the reflection beam. Therefore, the optical pickup apparatus of the present invention is effective in enabling the manufacture of an inexpensive, small-size optical pickup apparatus.